Certain vehicles have motorized lifts for raising and lowering large objects to facilitate loading them into and unloading them from space inside the vehicle. One example of such a device is a wheelchair lift. Various models of wheelchair lifts are commercially available. One type comprises a platform that when placed generally horizontally either on the floor of a vehicle or on ground adjacent the vehicle allows a wheelchair to be rolled onto and off it. With a wheelchair having been placed on the platform, the platform can be raised and/or lowered to move the wheelchair to and from floor- and/or ground-level. The lift is motorized, comprising a prime mover and associated mechanism for raising and lowering the platform. When unoccupied, the platform can be operated to a stowed position in preparation for the vehicle to be driven.
A motor vehicle having a wheelchair lift is subject to U.S. government regulations, as specified in FMVSS. One requirement mandates that the vehicle be rendered immovable when the lift is out of stow. One means for compliance with that requirement comprises automatically operating a vehicle's park brake to apply a holding or locking force to vehicle wheels when the vehicle is stopped thereby preventing the vehicle from moving.
In certain vehicles the park brake uses an on-board pressurized air supply to keep the park brake at each wheel from otherwise being automatically applied by a device, sometimes called a SAAR (meaning spring actuated, air released) for short. Application of a park brake occurs when the air supply to the SAAR is shut off, allowing the spring force of the SAAR to be effective to lock the wheels. Pressurized air must be applied to the SAAR in order to release the park brake.
The driver can apply and release the park brake by operating a “push-pull-double-check” (PPDC) valve, sometimes referred to as a “park brake knob”. For compliance with applicable wheelchair lift regulations, a vehicle may have a solenoid-operated interlock valve for causing the compressed air supply to the park brake knob to be shut off when the wheelchair lift is out of stow. A supply valve is caused to open by compressed air pressure being applied through the interlock valve to a pilot port when a switch or sensor signals that the wheelchair lift is in stow, thereby enabling the supply valve to open and pass compressed air from a supply port to a delivery port so that the park brake knob can apply and release the park brake.